The present invention relates to a color cathode-ray tube, and more particularly, to a color cathode-ray tube in which a shadow mask is supported by elastic supporting members for compensating a deviation in beam landing attributable to thermal expansion of the shadow mask or a face panel.
In general, a color cathode-ray tube is provided with an envelope that includes a funnel and a substantially rectangular face panel having side wall sections on the peripheral edge portion of its effective section. A phosphor screen, including three phosphor layers that individually emit light of three different colors, blue, green, and red, is formed on the inner surface of the effective section of the face panel. In the envelope, a substantially rectangular shadow mask is opposed to the inside of the face panel. Provided in a neck of the funnel is an electron gun that emits three electron beams.
The electron beams emitted from the electron gun are deflected by a deflecting device, which is attached to the outside of the funnel, and are used to scan the phosphor screen in the horizontal and vertical directions through the shadow mask. Thereupon, a color image is displayed on the phosphor screen.
The shadow mask serves to sort the three electron beams from the electron gun by color so that they can correctly land on the three phosphor layers. The mask includes a substantially rectangular shadow mask body having a large number of electron beam apertures and a substantially rectangular mask frame attached to the peripheral portion of the mask body. At least three side wall sections of the mask frame are supported on the side wall sections of the face panel by means of elastic holders, individually, so that the shadow mask body faces the phosphor screen at a predetermined distance therefrom. Each holder has one end portion fixed to the mask frame and the other end portion anchored to a stud pin on the inner surface of each corresponding side wall section of the face panel.
In the color cathode-ray tube having the shadow mask, 30% or less of the electron beams emitted from the electron gun reach the phosphor screen through the electron beam apertures in the shadow mask body, while about 70% strike against the mask body. Thus struck by the electron beams, the shadow mask is heated and undergoes thermal expansion. When a high-brightness image is displayed, in particular, the relative positions between the electron beam apertures and the phosphor screen are shifted by thermal expansion of the mask body and the mask frame. Accordingly, electron beam spots shaped by the mask body cannot strike against or land on the phosphor layers of desired colors, so that the color purity is lowered inevitably.
Such color purity deteriorations during the operation of the color cathode-ray tube mainly include one attributable to the thermal expansion of the shadow mask body and one attributable to the thermal expansion of the mask frame.
The color purity deterioration attributable to the thermal expansion of the mask body is observed in the initial stage of high-brightness image display, and the electron beam landing position is shifted from a predetermined position toward the center of the phosphor screen in the radial direction thereof. This is caused by a doming effect such that the shadow mask body, having a small thermal capacity, bulges toward the phosphor screen, since it is heated while the mask frame, having a larger thermal capacity, is hardly heated.
On the other hand, the color purity deterioration attributable to the thermal expansion of the mask frame is caused as the electron beam landing position is shifted radially outward from the predetermined position on the phosphor screen. In this case, the doming effect of the shadow mask body is reduced as heat from the mask body is transmitted to the mask frame so that the external size of the mask frame is enlarged, while the peripheral portion of the mask body is pulled by the mask frame.
In order to restrain the color purity deterioration attributable to the thermal expansion of the shadow mask body, the shadow mask body itself should preferably be formed of a material with a low thermal expansion coefficient. In this case, however, a problem may possibly be aroused by the difference in the degree of thermal expansion between the shadow mask body and the mask frame that is attributable to the difference in the thermal expansion coefficient. If the shadow mask body is formed of a low-expansion material and is pulled beyond its thermal expandability by the thermal expansion of the mask frame, however, the mask body can be prevented from extending to the degree corresponding to the thermal expandability of the mask frame by giving an appropriate elasticity to a fixing portion between the mask body and the mask frame or by considering some other countermeasure. Accordingly, the color purity deterioration attributable to the thermal expansion of the mask frame can be also restrained to some extent. In the case of a high-precision color cathode-ray tube used in a computer display or the like, therefore, the shadow mask body is often formed of a low-expansion material, such as invar, to cope with its thermal expansion.
It is known that the color purity deterioration attributable to the thermal expansion of the mask frame can be corrected by suitably shaping the holder. According to this arrangement, the holder is formed of a belt-like member, which is obtained by bending a belt-shaped metal plate, and a bimetal member fixed to the mask frame. The mask-frame side of the bimetal member serves as a lower-expansion member, and the belt-like member side as a higher-expansion member. A fixing portion between the bimetal member and the mask frame is situated nearer to the phosphor screen than a fixing portion between the bimetal member and the belt member in the direction of the tube axis.
When heat from the mask frame is transmitted to the bimetal member during the high-brightness image display, according to this arrangement, the holder is tilted by the thermal expansion of the higher-expansion member, so that the angle of engagement between the holder and the stud pin changes. As the holder moves in this manner, the shadow mask body moves toward the phosphor screen, and the original electron beam apertures are corrected so as to be situated on the paths of the electron beams. Thus, the color purity deterioration is restrained.
In the case where the low-expansion material is used for the shadow mask body, as mentioned before, however, the color purity is lowered by the change of the ambient temperature. This is caused by the difference in the thermal expansion coefficient between glass as the material of the face panel and the low-expansion material, such as invar, for the shadow mask body.
More specifically, the color cathode-ray tube is shipped after it is adjusted so that optimum electron beam landing is ensured during its manufacturing processes. If the working temperature is different from the ambient temperature for the adjustment operation, then it is concluded that the ambient temperature is changed. If the ambient temperature for the adjustment operation and the working temperature for the cathode-ray tube are 20.degree. C. and 40.degree. C., respectively, a temperature difference of 20.degree. C. is produced for all components of the tube.
The thermal expansion coefficient of invar for the shadow mask body, which is 1.2.times.10.sup.-6 .degree. C., is about a tenth as high as that of glass, which is 10.times.10.sup.-6 .degree. C. Thus, the degree of thermal expansion of the face panel exceeds that of the shadow mask body, so that the electron beam landing position is deviated inward from the predetermined position in the radial direction of the phosphor screen. When the ambient temperature changes descendingly, on the other hand, the degree of shrinkage of the face panel exceeds that of the shadow mask body, so that the electron beam landing position is deviated outward from the predetermined position in the radial direction of the phosphor screen.
Although the holders with the conventional construction can be effectively used for the correction of the color purity deterioration attributable to the thermal expansion of the mask frame, they accelerate the color purity deterioration attributable to the change of the ambient temperature. More specifically, the holders shift the electron beam landing position inward in the radial direction of the phosphor screen by moving the shadow mask body toward the phosphor screen. When the ambient temperature changes ascendingly, the bimetal member also undergoes thermal expansion, and the radially outward movement of the phosphor screen by the thermal expansion of the panel and the radially inward movement of the beam landing position are caused simultaneously. In consequence, deviations in the electron beam landing increase.
On the other hand, if the bimetal member of each holder is located on the other side so that the shadow mask body is shifted in the correcting direction against the change of the ambient temperature, in order to correct the color purity deterioration attributable to the ambient temperature change, then the color purity deterioration attributable to the thermal expansion of the mask frame will be accelerated.
Thus, deviations of the electron beam landing position, which cause the color purity deteriorations, occur in one direction when the mask frame is thermally expanded during the operation of the color cathode-ray tube, and in the opposite direction when the ambient temperature is changed. Accordingly, the holders that use bimetal cannot simultaneously correct the color purity deteriorations of the two kinds.